Abstract
New sodium lithium rare earth carbonates Na
2LiLn(CO
3)
3 (
Ln
=
Eu–Er
, Yb, Lu, Y) are prepared by hydrothermal synthesis and characterized by single crystal X-ray diffraction, thermal analysis, IR, Raman and NMR spectroscopy. Crystal cells are cubic,
a
≈
14.4
Å
, space group
F
d
3
¯
m
(No. 227),
Z
=
16
. For Na
2LiYb(CO
3)
3, refinements yield
R
(
F
2
)
=
0.068
and
R
w
(
F
2
)
=
0.202
for 235 unique reflections. The structures are built up from [Li(1)O
6] octahedra, [Li(2)O
4] tetrahedra and [(Na
2/3Ln
1/3)O
8] polyhedra. NMR spectroscopy shows that Li
+ cations are disordered and present a high mobility in their tetrahedral and octahedral coordination polyhedra. Na
+ and Ln
3+ cations are also disordered; however, Na
+ cations adopt preferentially a six-fold coordination. Crystals of Na
2LiEu(CO
3)
3 are highly resistive. Raman and IR lines, associated with carbonate group vibrations, are very broad. Structural correlations exist with Na
3Mg(CO
3)
2Cl northupite and AB
2X
6 pyrochlores.